They studied a material with a low refractive index bordered by a layer with a high refractive index.
Most optical fibers have a core of pure silica and a cladding layer that is doped with ions to change its refractive index.
There's a mismatch between the refractive index of water and air.
Each cell type has a unique diffraction signature that depends on its size, shape, and an optical quality called refractive index.
British Dictionary definitions for refractive index
(physics) a measure of the extent to which radiation is refracted on passing through the interface between two media. It is the ratio of the sine of the angle of incidence to the sine of the angle of refraction, which can be shown to be equal to the ratio of the phase speed in the first medium to that in the second. In the case of electromagnetic radiation, esp light, it is usual to give values of the absolute refractive index of a medium, that is for radiation entering the medium from free space ν, μ